1. Field of the Invention
This invention pertains to the food industry. More particularly, it relates to a method of reducing reverse osmosis membrane fouling and improving reverse osmosis dewatering of an aqueous caffeine stream in the manufacture of coffee and coffee by-products.
2. Description of the Prior Art
In the process of manufacturing coffee, to make the decafinated version, the coffee bean is subjected to various treatments that results in the caffeine being extracted from the bean in an aqueous solution along with various oils and acids. This solution, usually 0.3% caffeine, is then dewatered to produce various concentrates in ranges from 2%, 4% and 6% caffeine and coffee bean extract that are thereafter useful as feed stock in subsequent processes that ultimately result in recovered caffeine used in the chemical industry.
Dewatering is performed using reversed osmosis (RO) units. RO units generally comprise elongated spiral-wound cartridges used either singularly or in groups. The cartridge is commonly constructed of one or more generally rectangular envelopes made of opposed sheets of a proprietary plastic film, sealed about three sides, that passes water molecules thereacross without passing other larger molecules, such as caffeine and other materials, held apart by an interior layer of polysulfone and whose non-sealed edges are separately attached to the sides of an elongated slit formed in the wall of a centralized product tube. The envelope or envelopes are thereafter spiral-wound about the center product tube along with a layer or layers of an open web or stream called "brine channel webbing", to maintain the outer surfaces of a particular envelope free from direct contact with an overlying or underlying layer of membrane envelope, to a tight cylinder about the center product tube.
In use, one end of the spiral-wound cartridge is sealed to prevent water from exiting the layer of webbing and the other end is connected to a source of aqueous caffeine solution. The pressure of the feed solution is raised to exceed the osmotic pressure, the aqueous feed enters into the layer taken up by the brine channel webbing between the rolled envelopes of reversed osmosis material to force the water into the center of the envelopes where it moves by migration and pressure along the layer of polysulfone to enter through the slits into the center product tube and exit the cartridge as clear, fresh water thereby dewatering the aqueous caffeine solution. The feed stream to the cartridge is either referred to as "feed stock" while the filtered water produced from the cartridge is called the "permeate" stream.
Other reverse osmosis units are tubular wound as is well known in the art. Their operation is similar in concept to the spiral-wound units. Reverse osmosis differs dramatically from other forms of separation such as filtration or microfiltration. These latter two processes utilize a paper-like media that have holes formed therethrough to allow passage of water molecules or other molecules that are designed to be removed from the feed stock. To aid separation of the components in filtration and ultrafiltration processes, it is common to complex one or more of the feed stock components to prevent it from passing through the filter. To be successful, it is important to use a stoichiometric quantity of the complexing agent to insure complete complexing of all the component that is to be isolated.
In the reverse osmosis process, the media is a semi-permeable membrane that does not have holes like that of the microporous filter media. The feed stock is introduced to the membrane and the pressure raised to a level in excess of the osmotic pressure of the stock to allow water molecules to pass through the membrane into the pure water (permeate) outlet stream.
Presently, the feed stock (approximately 0.3% aqueous caffeine and coffee bean extract solution) is fed to the reverse osmosis (RO) unit under conditions such as 200 pounds per square inch and at a temperature of about 60.degree. Centigrade. Water is drawn off the RO unit as the permeate stream thereby producing a higher concentration of caffeine in the effluent that is the product of the process.
In present-day RO operation, insoluble materials in the feed stock such as silt and colloidal materials such as iron oxide, silicates, such as those of calcium, barium, strontium, magnesium, manganese and other cations react with anions such as sulfates, carbonates, chlorides, etc. and form a foulant or scale on the RO permeable membrane and inhibit the efficiency of the dewatering process. Other foulants found to interfere with the efficiency of the dewatering process are acids such as chlorogenic acid, caffeic acid, quinic acid, citric acid, malic acid, tartaric acid, oxalic acid, pyruvic acid, acetic acid, propionic acid, butyric acid, and valeric acids as well as aldehydes such as acetylaldehyde, propyl aldehyde, butyl aldehyde, and valor aldehyde; ketones such as ketone, didactyl ketone; ketone-alcohols such as acetol, and acetoin, esters such as methyl formate, methyl acetate, methyl propionate, and propyl formate; sulfides such as hydrogen sulfide, dimethyl sulfide, methyl sulfide, mercaptan, thiophene, and cylics such as furan, methyl furan, pyridine, amino acids, amines such as trimethylamine, dimethylamine and ammonia; cellulose both hydrolyzed and non-hydrolyzed; hydrocarbons such as isoprene; unsaturates such as acrolein, caramel, mineral ash, trigonelline and associated processed oils.
Layers of inorganic, organic and other foulant materials, soluble or insoluble in water at varying pH, tend to deposit, after a period of use, on the surface of the semi-permeable membranes and substrates, both supporting and non-supporting. Such deposits seriously impeding the membrane performance by reducing the product flow rate and often the salt rejection rate and thereby impede the processing efficiency. In general, it is believed that if particulate or colloidal matter could be kept suspended, insoluble foulants could be controlled.
Scale-forming type of foulants do not occur if chemical reaction and reaction product precipitation can be prevented. Chemical reaction of the scale-forming reactants can be inhibited and/or controlled by the formation of intermediate products that remain soluble or suspended in the aqueous matrix. Further, precipitation of water soluble material does not occur unless the conditions allowing super-saturation occur.
As the RO unit becomes fouled, the pressure difference between the feed and permeate streams must be raised to maintain adequate flow rates. RO units have a maximum tolerable pressure differential before structural failure is encountered. After this maximum pressure is reached, the process flow rate can only be maintained by shutting down the RO unit and physically cleaning the membranes. This requires draining the feed stock from the unit, filling it with a cleansing solution, cycling the solution through the unit to remove the foulants and scale, draining and rinsing the unit and then recharging the feed stock to the clean unit. Each time this occurs the product stream is interrupted resulting in lost process time, increased labor costs and lost profits.
The prior art has dealt with caffeine in coffee with mixed results. In U.S. Pat. No. 4,160,042, there is disclosed a means of removing caffine from coffee by passing the coffee solution through a bed of ground carob pods so that the caffeine absorbs on the pod material and is removed from the coffee. The carob pods are then lixiviated with hot water to remove the caffeine. This process, however, produces the exact feed stock that the instant invention uses to dewater the caffeine-lixiviated solution.
It is to be noted that this same patent cites a French Patent Application No. 2,231,407 as disclosing the fixing of caffeine on a macromolecular substance in the liquid state and subxequently separating the aggregate so formed by ultrafiltration on a semipermeable membrane. This is a disclosure of mixing a solid with a liquid, to have the solid absorb caffeine from the liquid, then separate the caffeine-complexed solid by filtration. The disclosure also reports that the method has not been accepted for industrial application.
These prior art methods utilize sufficient amounts of macromolecular substances to reach stoichiometric proportions of the caffeine in the feed stock. Such large amounts of material require large equipment to lixiviate the caffeine and then still produce a watered feed stock that requires dewatering before it becomes commercially feasible. This increased the cost of processing the feed stock, prior to dewatering it, constitutes a major impediment in the production of commercial concentrations of caffeine.